1. Field of the Invention
Electromagnetic interference (“EMI”) shielding gasket assembly for stuffing tubes in ships.
2. Background Information
Modern ships, watercraft, buildings, and vehicles often include electronic equipment that is used within the interior, but often has antenna or other functional parts extending outside. For this reason, there must be an electrical communication from the outside that extends through the wall into the interior. The need for watertight seals often require that special allowances be made. Thus, in conventional construction, ships, buildings, vehicles, and the like use stuffing tubes where electrical communication lines (such as electrical cables) extend through the wall, deck or a bulkhead.
In general, a stuffing tube comprises a tubular or cylindrical shell inserted through a deck or bulkhead and attached there via a weld, adhesive, or fastener. The cable extends from the exterior structure (such as mast or antenna) through the stuffing tube and to the electronic equipment. Seals are used around the cable in the interior of the tube, as well as various machined parts that compress the seal. The seals resist water and air flow between the interior and exterior of the ship.
Shipboard waterproof transition fittings, kick pipes or stuffing boxes are provided to route cables between topside and below deck or between bulkheads defining inter-compartmental spaces. Typically a metallic boundary is provided to shunt environmental EMI or electromagnetic pulse (“EMP”) signals to a metallic boundary, such as the ship deck. EMI (also sometimes called radio-frequency interference or “RFI”) is a disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source. The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit. The source may be any object, artificial or natural, that carries rapidly changing electrical currents, such as the cable extending through the stuffing tube. The ship deck may be envisioned as a system ground plane, for grounding above deck conduit or cable. Without proper grounding to the ground plane, any external substantial EMI source or EMP pulse may penetrate the metallic boundary (deck), enter below decks, and harm susceptible, sensitive electronic equipment.
In FIG. 1, illustrating prior art, a metallic fitting, such as a stuffing tube, is seen to engage and direct a cable to the deck or bulkhead. Moreover, it is seen that conduit insulation is stripped from the fitting to expose at least some of the metallic shield jacket, such that it is inside the fitting. Finally, it may be seen, in the prior art, that a metallic substance, such as a steel wool type compressible material having a multiplicity of filaments, may be jammed in to a stuffing tube space beneath the gland nut and the gland nut tightened down. Tightening the gland nut will squeeze some of the air out, and jam the filaments of the steel wool down, so there are multiple electrical paths between the exposed shielding jacket and the metallic stuffing tube. Thus, a strong EMI/EMP generated signal or current on the shield gasket is grounded to the ship deck.